Flexible pipes comprising a carcass and an inner sealing sheath (also called an inner liner) are well known in the art and are generally used for the transportation of oil and gas products over long distances and often at elevated temperatures, such as above 60° C. or more.
The flexible pipes for offshore use are often unbounded pipes. The term “unbounded” means in this text that at least two of the layers of the pipe are not bonded to each other. Often unbounded flexible pipes comprise an internal sheath which forms a barrier against the outflow of the fluid which is conveyed through the pipe, and one or more armouring layers on the outer side of the internal sheath (outer armouring layer(s)). The flexible pipe may comprise additional layers such as a carcass which is an inner armouring layer to prevent the collapse of the internal sheath. An outer sheath may be provided with the object of forming a barrier against the ingress of fluids from the pipe surroundings to the armouring layers. In practice the pipe will comprise at least two armouring layers, which are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
Typical unbounded flexible pipes are e.g. disclosed in WO0161232A1, U.S. Pat. Nos. 6,123,114 and 6,085,799.
The above-mentioned type of flexible pipes is used, among other things, for off-shore as well as some on-shore applications for the transport of fluids and gases. Flexible pipes can e.g. be used for the transportation of fluids where very high or varying water pressure exists along the longitudinal axis of the pipe, such as riser pipes which extend from the seabed up to an installation on or near the surface of the sea, pipes for transportation of liquid and gases between installations, pipes which are located at great depths on the seabed, or between installations near the surface of the sea.
In traditional flexible pipes, the one or more outer armouring layers are most often in the form of helically wound steel wires e.g. shaped as profiles, where the individual layers may be wound at different winding angle relative to the pipe axis.
When using such prior art flexible pipes for transportation of aggressive gases, raw oils and similar fluids, the pipe should be constructed to resist corrosion of the armouring layers. This may e.g. be provided by protecting the armouring layer from diffusion of aggressive gasses through the internal sheath e.g. by applying a gas barrier layer to prevent the diffusion e.g. as described in WO05028198.
The internal sheath should be chemically stable and mechanically strong even when subjected to high temperatures. Furthermore, the internal sheath should be manufactured in one piece since repair, welding or other types of connecting methods are not acceptable for internal sheaths in offshore pipelines. The internal sheath is therefore normally produced by continuous extrusion of a polymer. A number of polymers are presently used for the production of internal sheaths, such as Polyamide-11 (PA-11), polyethylene (PE) and Polyvinylidene difluoride (PVDF).
These materials shall fulfil the combined requirements of e.g. heat stability, resistance to crude oil, seawater, gases, mechanical fatigue, ductility, strength, durability and processability. The internal sheath material is normally selected on a case-to-case basis after careful investigation of the conditions for the planned installation. Here, cross-linked polyethylene may in many cases prove to fulfil the requirements.
Additionally, the interest in use of internal sheaths in corrosive applications with high concentrations of carbon dioxide and/or hydrogen sulphides is increasing. Furthermore, polyamides are susceptible to hydrolysis. However, the permeability of gases increases with temperature, and polyethylene has a relatively high permeability to gases. Thus, permeation of gases like methane, carbon dioxide and hydrogen sulphide may in some cases be prohibitive for use of cross-linked polyethylene internal sheaths at high temperatures unless the internal sheath is provided with an additional gas barrier foil between the fluid to be transported and the polyethylene material.
In EP 487 691 it has been suggested to use an internal sheath of cross-linked polyethylene. An internal sheath with such cross-linked material has proved to be highly improved compared to internal sheaths of the similar non-cross-linked (thermoplastic) material.
In order not to degrade the material, the process in the prior art of producing an internal sheath is carried out in two steps, first the material in non-cross-linked form is manufactured by extrusion, and afterwards the material is cross-linked. When the material is cross-linked, it is difficult to change its shape without degrading the material.
The cross-linking step disclosed in EP 487.691 is very cumbersome and time and space demanding, and furthermore it has been found that the cross-linking degree obtainable by using this method is extremely low and not homogenous through the layer.
In the co-pending application WO03078134 filed by applicant is disclosed a process of producing a flexible pipe e.g. comprising a carcass and an internal sheath of a cross-linked polyethylene material. The process comprises the steps of shaping a polyethylene material comprising a peroxide by extrusion onto the carcass in an extrusion station and cross-linking the extruded polyethylene material by exposing the extruded polymer material to electromagnetic waves, selected from the group consisting of infrared radiation and microwave. By this method it has been found to be possible to obtain a much higher degree of cross-linking, e.g. in area of about 80%. However, it is still desirable to obtain an even higher cross-linking degree of the polyethylene material. Furthermore, the method disclosed in WO03078134 for the production of an internal sheath has been found to result in a polyethylene internal sheath with an inhomogeneous degree of cross-linking in the thickness direction of the internal sheath.